1. Field of the Invention
The present invention relates to a swing rotor for a centrifugal separator, a micro-plate adapter for mounting a micro-plate including a sample onto the swing rotor for a centrifugal separator, and a method for centrifugal separation.
2. Description of the Related Art
First, a micro-plate will be described with reference to FIG. 1. A micro-plate 4 is used in a manner such that it is put into a centrifugal separator after an active reagent has been dropped on a body fluid such as blood, or it is used for various experiments in a tissue culture field or a genetic engineering field including a centrifuge separation process as an intermediate step. Such a micro-plate 4 is generally formed by molding with plastic material such as polystyrene or polypropylene. The micro-plate 4 is a box-like vessel about 130 mm long, about 90 mm wide and about 10 to 50 mm high, and a large number of small concave sample injection hole portions 5 for injecting a sample are provided so as to be regularly aligned vertically and horizontally in the upper surface portion thereof. A notch portion 8 is provided in the lower portion of a box-like external wall 7 of the micro-plate 4 in consideration of laying on another micro-plate 4. The size of this notch portion 8 is substantially corresponding to the size of the upper surface portion of the micro-plate 4 so as to prevent positional displacement from occurring when such micro-plates 4 are laid on each other. If this notch portion 8 is not provided in a position lower than a plate bottom surface 9 of the micro-plate 4, it is impossible to prevent the positional displacement when such micro-plates 4 are laid on each other. Therefore, the box-like external wall 7 of the micro-plate 4 is extended to a position lower than a plate bottom surface 9.
Next, a rotor for a centrifugal separator for centrifuging the above-mentioned sample in the micro-plate 4 will be described. Such a rotor for a centrifugal separator is disclosed in, for example, Japanese Utility Model Examined Publication No. Sho 57-934, and it will be described here with reference to FIGS. 2 and 3. FIG. 2 is a perspective view of the appearance of a swing rotor, and FIG. 3 is a perspective view of the appearance of a metal adapter mounted on the swing rotor of FIG. 2. In FIG. 2, the rotor is constituted by a rotor body 1 and a bucket 2. A rotation force is given to the rotor body 1 by a not-shown centrifugal separator, and the bucket 2 swings outward by a centrifugal force caused by this rotation force so as to give centrifugal acceleration to a sample held in the bucket 2.
To use such a swing rotor for separating a sample contained in the micro-plate 4, generally, a metal adapter 3 is mounted on the bucket 2. The adapter 3 has an outer size so as to be held by the bucket without looseness, and further has bent portions 12 and 13 for holding the outer circumference of the micro-plate 4 in order to eliminate looseness between the adapter 3 and the micro-plate 4 when the adapter 3 holds the micro-plate 4. The adapter 3 is manufactured by finishing a metal plate such as a stainless steel plate or an aluminum plate, and a bottom portion 11 thereof is made flat.
FIG. 1 shows the structure in which the aforementioned micro-plate 4 is mounted on the adapter 3. As mentioned above, the box-like external wall 7 of the micro-plate 4 is extended to a position lower than the plate bottom surface 9, and the bottom portion 11 of the adapter 3 is made flat, so that there is a gap portion 10 between the micro-plate 4 and the adapter 3. In such a state, the adapter 3 is usually used at the rotational speed of about 2,000 rpm, and the maximum centrifugal acceleration of 700 Xg.
Therefore, conventional rotors available on the market have the maximum rotational speed of 2,000 rpm and the maximum centrifugal acceleration of about 600 to 800 Xg, which belongs to a range in which no damage occurs in micro-plates.
As the usage and field of application intended by the present invention, it is directed to the improvement of efficiency in studies relating to DNA or RNA which has been studied prosperously in a genetic engineering field and so on. Centrifugal separation of the DNA as a sample is one of important processes in the procedure of DNA sequencing in such a field. Particularly in DNA recovery methods through ethanol precipitation performed by adding a proper quantity of ethanol or the like to a solution including DNA, a higher recovery percentage has been desired. The recovery percentage, however, was about 75% with a conventional rotor having the maximum rotational speed of 2,000 rpm, and the maximum centrifugal acceleration of about 600 to 800 Xg.
In order to increase this recovery percentage, it is necessary to perform separation under a higher centrifugal acceleration, and therefore centrifugal separation has been performed at the rotational speed of about 12,000 rpm (about 10,000 Xg) for about 10 minutes by using a plastic micro-tube (test tube) of about 0.2 ml to 2 ml.
However, since micro-tubes are handled one by one in this operation, the operation is troublesome. Further, since micro-tubes not micro-plates are used, only about 48 tubes at maximum can be treated in one driving because of the limitation of an apparatus in centrifugal separation.
Recently, various inspections of symptoms about the health of human bodies or experiments in a tissue culture field have been performed with a micro-plate flourishingly, and it is necessary to improve the efficiency in centrifugal separation process required in an intermediate process of the inspections or experiments. The improvement of the efficiency in a centrifugal separation process can be attained by increasing the rotational speed of a rotor to thereby increase the centrifugal acceleration.
However, if the rotational speed of the rotor structured as above thus is increased to improve the efficiency, a group of the sample injection hole portions 5 of the micro-plate 4 are broken due to collapse from a border portion 6 between the group of the sample injection hole portions 5 and the box-like external wall 7. Thus the desired separation, cannot be attained. Because the gap portion 10 exists between the plate bottom portion 9 of the micro-plate 4 and the bottom portion 11 of the adapter 3 when a centrifugal load caused by the centrifugal acceleration is given to the micro-plate 4, the sample injection hole portions 5 are bent to the gap portion 10 side by the centrifugal load. As a result of the centrifugal loud a large bending moment is given to the border portion 6 between the group of the sample injection hole portions 5 and the box-like external wall 7, so that the border portion 6 is broken. According to experiments effected by the present applicant, ordinary micro-plates 4 available on the market were examined, and as a result, the border portion 6 was broken at about 1,000 Xg (1,000 times as high as the gravitational acceleration). Generally, polystyrene is often used as the material for the micro-plate 4. However none of the reasons for the above-mentioned damage is that polystyrene is weak in the property of strength.
Since a rotor for a centrifugal separator generates high centrifugal acceleration, the lighter a subject to be separated including the adaptor 3 held by the bucket 2 is, the smaller the centrifugal load can be made, and it is therefore advantageous to reduce the burden of the rotor body and the bucket. In addition, buckets are disposed symmetrically with respect to the rotation axis, so that it is necessary to consider the mass balance of opposite buckets and the position balance of the center of gravity thereof. If the micro-plate is held and rotated in a state in which it is displaced in position, the rotor rotates while vibrating greatly so that the centrifugal separator may be broken to make it impossible to attain the desired centrifugal separation.